Solar thermal collection apparatus and methods

ABSTRACT

A solar thermal collector includes a receptacle and a fluid conduit. The receptacle is evacuated to a subatmospheric pressure. The receptacle includes a window and a reflector facing the window. The window and the reflector are exposed to the subatmospheric pressure in the receptacle. The fluid conduit extends through the receptacle between the window and the reflector. The reflector concentrates solar radiation passing through the window onto the fluid conduit.

BACKGROUND

Solar thermal collectors collect heat by absorbing solar radiation.Solar thermal collectors come in a variety of different types, includingflat-plate collectors, evacuated tubes, and solar concentratingcollectors. A flat plate solar thermal collector has a broad flat platesolar radiation absorber, whereas an evacuated tube collector containsan absorber within an evacuated tube. A concentrating collector includesa reflector that focuses radiant energy onto a localized solar radiationabsorber. In both cases, the solar radiation absorber converts the solarradiation into heat energy that typically is transferred to acirculating heat transfer fluid. Solar thermal collectors may beincorporated into stationary installations or into installations thattrack solar azimuthal position.

DESCRIPTION OF DRAWINGS

FIG. 1A is a diagrammatic cross-sectional view of an example of a solarthermal collector.

FIG. 1B is a diagrammatic perspective view of an example of a reflector.

FIG. 2 is a diagrammatic cross-sectional view of an example of a fluidconduit.

FIG. 3 is a flow diagram of an example of a method of manufacturing asolar thermal collector.

FIG. 4A is a diagrammatic cross-sectional view of an example of a solarthermal collector.

FIG. 4B is a diagrammatic top view of an example of a fluid conduit.

FIG. 5 is a diagrammatic top view of an example of a solar thermalcollector.

FIG. 6 is a flow diagram of an example of a method of using a solarthermal collector.

DETAILED DESCRIPTION

In the following description, like reference numbers are used toidentify like elements. Furthermore, the drawings are intended toillustrate major features of exemplary embodiments in a diagrammaticmanner. The drawings are not intended to depict every feature of actualembodiments nor relative dimensions of the depicted elements, and arenot drawn to scale.

At least some of the examples that are described herein provide solarcollection apparatus and methods that reduce manufacturing costs,improve reliability, and improve efficiency through the incorporation ofa reflector and a fluid conduit into an evacuated receptacle. Theseexamples eliminate the plurality of individual evacuated glass tubesthat typically are used in prior solar panel designs to contain theabsorber elements. In this way, these examples are able to achieve lowconvective losses while avoiding the increased manufacturing complexityand cost associated with such evacuated tube based solar thermalcollectors. At least some of these examples may be assembled withsubstantially fewer fluid connections as compared toevacuated-tube-based solar thermal collectors, which require a fluidconnection for each tube. By reducing the number of fluid connectionsand their associated convective losses, the operating performance ofthese examples are expected to be higher than evacuated tube based solarthermal collectors. In addition, by containing the fluid connectionswithin the evacuated space, these examples enable convective losses andinsulation requirements to be significantly reduced.

FIG. 1A shows an example of a solar thermal collector 10 that includes areceptacle 12 and a fluid conduit 14 (also referred to as an absorber).The receptacle 12 is evacuated to a subatmospheric pressure and includesa window 16 and a reflector 18 facing the window. The window 16 and thereflector 18 are exposed to the subatmospheric pressure in thereceptacle 12. The fluid conduit 14 extends through the receptacle 12between the window 16 and the reflector 18. The reflector 12concentrates solar radiation passing through the window 16 onto thefluid conduit 14. In some examples the fluid conduit 14 defines agenerally cylindrical flow path with a circular cross-section.

The reflector 18 may be any type of reflector that concentrates incomingsolar radiation onto the fluid conduit 14, including imaging solarconcentrators (e.g., cylindrical and parabolic concentrators) andnon-imaging solar concentrators that use non-imaging optics geometriesto concentrate solar radiation onto the fluid conduit 14. In theillustrated example, the reflector 18 includes a pair of concaveradiation-reflective surface portions 20, 22 that meet along alongitudinal axis 24 (represented by the center of the dashed circle) ina longitudinal plane about which the concave radiation-reflectivesurface portions 20, 22 are symmetrical. In other examples, the concaveradiation-reflective surface portions are asymmetrically offset onopposite sides of the longitudinal plane. In some examples, the pair ofconcave surface portions form a non-imaging concentrator collector(NICC) trough (see FIG. 1B). In these examples, the concave surfaceportions are curved with respective parabolic profiles that are inclinedat the same angle with respect to the longitudinal plane and focusincoming solar radiation to a common confocal solar concentration line25 that lies in the longitudinal plane and is parallel to thelongitudinal axis 24, as shown in FIG. 1B.

In the illustrated embodiment, the reflector 18 is integrallyincorporated into a base 26 of the receptacle 12. In some examples, thebase 26 is a unitary metal structure that comprises the reflector 18.The unitary metal structure may be formed, for example, by a metalextrusion process (e.g., an aluminum extrusion process) or a metalstamping process (e.g., a steel stamping process). In other examples,the base 26 is an extruded or molded plastic piece and the reflector 18is made of a metal (e.g., an aluminum or silver film) that is bonded tothe plastic base. In some examples, a getter (e.g., a zirconium basedgetter) is applied to the reflector 18 in order to capture gases thatoutgas from or leakage into the receptacle 12 and thereby preventoxidation of the receptacle (which can serve as a source of emissiveloss) after the receptacle 12 has been sealed and evacuated.

The fluid conduit 14 extends adjacent the reflector 18 along theconfocal solar concentration line 25 of the reflector 18. The fluidconduit 14 may be any type of structure through which a heat transferfluid (e.g., air, water, oil such as synthetic paraffin oil, and supercritical carbon dioxide) can be circulated. In some examples, the fluidconduit 14 is a hollow cylindrical metal tube 28 (e.g., an aluminum orcopper tube) that includes an outer surface and an inner surface. In theillustrated example, the outer surface of the fluid conduit 14 carries asolar radiation absorbent coating 30. The coating 30 typically includesa solar selective absorbent layer (e.g., aluminum nitride,metal-aluminum nitride cermets, or titanium oxide layer) and,optionally, one or more additional layers, including an overlyingantireflection layer that passes solar radiation to the radiationabsorbent layer with minimal reflections and an underlying stabilizinglayer under the radiation absorbent layer. In some examples, the solarradiation absorbent coating 30 targets maximum absorption of solarthermal energy and rejects solar spectra outside this range, whilemaintaining an ideal emissivity of less than 6% at 180° Celsius. In someexamples, the inner surface of the fluid conduit 14 is textured withsurface features that disturb the flow of fluid through the fluidconduit 14 to reduce laminar flow and increase heat transfer to thecirculating fluid. In some of these examples, the textured inner surfaceof the fluid conduit 14 is formed by creating patterns (e.g., helicalpatterns) of ribs and grooves in the inner surface of the fluid conduitusing a knurling tool.

FIG. 2 shows an example 29 of the fluid conduit 14 that includesmultiple separate fluid channels 31 for concurrently conveying fluidthrough the receptacle 12. This example has particular utility forcirculating heat transfer fluids at high pressure (e.g., 10³ pounds persquare inch (PSI) or 6.89×10³ kilopascals (kPa), or higher), such assupercritical, carbon dioxide (i.e., carbon dioxide at a temperature andpressure greater than or equal to its critical temperature andpressure). In some examples, each of the fluid channels 31 has arespective channel diameter 33 between 0.75 millimeter and 0.25millimeter. In one example, each of the plurality of fluid channels 31has a channel diameter of 0.5±0.01 millimeter. In some examples, thefluid channels 31 form a matrix of parallel microchannels in a tubularstructure. In some of these examples, the channels may be formed in asolid aluminum tube by electrochemical oxidation as described inDelendik et al., “Aluminium oxide microchannel plates,” Nuclear PhysicsB—Proceedings Supplements, Volume 125, pages 394-399 (September 2003).In other examples, the fluid channels 31 may be formed by etching awaysacrificial fillers (e.g., sacrificial wires) that are embedded in atubular aluminum substrate.

Referring back to FIG. 1A, the window 16 passes solar radiation to thereflector 18. In the illustrated example, the window 16 includes atempered glass substrate 32 that has a top surface carrying a topantireflective coating 34 and a bottom surface carrying a bottomantireflective coating 36. The antireflective coatings 34, 36 may be anyof a variety of different types of antireflective coatings including,for example, porous antireflective coatings and textured multi-layersol-gel antireflective coatings. The antireflective coatings 34, 36reduce Fresnel losses and, together with the far-angle performance ofnon-imaging concentrator design of the reflector 18, mitigate blue-shiftand Brewster angle losses (whereby an increase in the amount ofreflected light due to the increased incident angle reduces performance)resulting from the presence of the window 16.

In the illustrated example, the base 26 of the receptacle 12 includes aperipheral groove 38 into which the window 16 is recessed such that thebottom edges of the window 16 are supported by the foot of the grooveand the top edges of the window are flush with the top surface of thebase 26. The window 16 is mounted to the base 26 with a connection thatmaintains the subatmospheric pressure in the receptacle 12. In examplesin which the base is formed of a metal (e.g., aluminum) and the window16 is formed of glass, the connection between the window 16 and the base26 may include a glass-to-metal seal (e.g., a solder based seal or alaser weldable glass or ceramic seal) that maintains the subatmosphericpressure in the receptacle 12.

FIG. 3 shows an example of a method of manufacturing the solar thermalcollector 10.

In accordance with the method of FIG. 3, the fluid conduit 14 isattached to the base 26 (FIG. 3, block 40). In some examples, the fluidconduit 14 is supported at opposite ends of the reflector 18 byrespective support structures in the base 26 such that the fluid conduitis suspended over the reflector 18. In some of these examples, one ormore struts may be added to the reflector 18 to provide additionalsupport for the fluid conduit 14 along the length of the reflector 18.

The window 16 is mounted to the base 26 to form the receptacle 12containing the fluid conduit 14, where the reflector 18 faces the window16 and concentrates solar radiation passing through the window onto thefluid conduit 14 (FIG. 3, block 42). As explained above, the window 16is mounted to the base 26 with a connection that is capable ofmaintaining a subatmospheric pressure in the receptacle 12.

The receptacle 12 is evacuated to a subatmospheric pressure, where thewindow 16 and the reflector 18 are exposed to the subatmosphericpressure in the receptacle 12 (FIG. 3, block 44). In some examples thereflector 18 is evacuated to a pressure of about 10⁻³ pascal (Pa). Insome examples, before the receptacle 12 is evacuated, the receptacle 12is purged of air using an inert gas (e.g., argon) in order to reduceconvective losses.

FIG. 4A shows an example of a solar thermal collector 46 that includes areflector 48, an example 50 of the fluid conduit 14, and an example 52of the window 16.

The reflector 48 includes an array of reflector elements 54, 56, 58, 60,62, 64, 66, 68 each of which corresponds to the reflector 18 describedabove. In the illustrated example, each of the reflector elements 54-68is formed from a respective pair of concave radiation-reflective surfaceportions, the concave radiation-reflective surface portions of eachreflector element 54-68 meet along a respective longitudinal axis in arespective longitudinal plane about which the concaveradiation-reflective surface portions are either symmetrically orasymmetrically disposed, and the respective longitudinal axes areparallel. Each reflector element 54-68 concentrates radiation passingthrough the window onto a different respective section of the fluidconduit 50.

Referring to FIG. 4B, the fluid conduit 50 includes parallel linearsegments 72, 74, 76, 78, 80, 82, 84, 86 each of which extends adjacent arespective one of the reflector elements 54, 56, 58, 60, 62, 64, 66, 68along a respective direction in a respective one of the longitudinalplanes that is parallel to the respective longitudinal axis. The fluidconduit 50 also includes curved segments 88, 90, 92, 94, 96, 98 thatinterconnect the linear segments 72, 74, 76, 78, 80, 82, 84, 86 todefine a serpentine fluid flow path adjacent the reflector 48.

Referring back to FIG. 4A, the window 52 corresponds to the window 16described above. In this example, the side walls of the reflectorelements 54-68 provide distributed support along the width of the window52 (e.g., every 39 millimeters in some examples). This support enablesthe thickness of the window 52 to be reduced. In some examples, thewindow 52 is formed of a tempered glass substrate that has a thicknessin the range of 1.5 to 3 millimeters.

FIG. 5 shows a top view an example of a solar thermal collector 100 thatincludes a set of six panels 102, 104, 106, 108, 110, 112 each of whichcorresponds to the solar thermal collector 46 (see FIG. 4A), where therespective reflectors 48 are demarcated by the dashed boxes. Theterminal ends of each of the fluid conduits 50 are connected to amanifold 114. The manifold and the fluid conduits 50 of the panels102-112 together define a recirculating fluid path for a heat exchangefluid. In some examples, the manifold is integrated into the panels,which avoids the need to provide separate insulation for the manifold.

Table 1 below shows a comparison of the relative sizes of components ofan exemplary evacuated-tube-based solar thermal collector and anexemplary tubeless solar thermal collector of the type shown in FIG. 1A.As shown in Table 1, the elimination of evacuated tubes, allows thedimensions of the reflector and the absorber in tubeless solar thermalcollector to be significantly reduced. This feature enables thethickness of the window 52 to be reduced significantly as compared totube-based designs, thereby reducing the losses and weight associatedwith thicker windows.

TABLE 1 Evacuated Tube Based Tubeless Solar Solar Thermal CollectorThermal Collector Orientation North-South North-South Acceptance Angle~60 ~60 [Degrees] Concentration 1.152 1.152 Reflector Height [mm] 108.0119.079 Reflector Width [mm] 202.63 39.80 Tube Viessmann none AbsorberDiameter [mm] 56 11 Tube Diameter [mm] 65 N/A Panel height 192 25

In some examples, the solar panel array shown in FIG. 5 is part of afixed solar thermal installation. In some of these examples, the solarpanel array is mounted at a fixed angle in a North/South configurationin which the longitudinal axes of the reflectors are fixed vertically ata tilt equal to latitude facing the sun perpendicular to its solar noonposition. In other ones of these examples, the solar panel array ismounted at a fixed angle in an East/West configuration in which thelongitudinal axes of the reflectors are similarly fixed but in ahorizontal fashion. Such an installation is advantageous in that it doesnot require any mechanical tracking because the non-imaging opticalrelationship between the reflectors and the fluid conduits provides anacceptance angle of up to +/−60° while concentrating between 1.05 and1.08 times the collected energy onto the solar radiation absorbing fluidconduits.

FIG. 6 shows an example of a method of using a solar thermal collector.

In accordance with the method of FIG. 6, a solar thermal collector isprovided (FIG. 6, block 120). The solar thermal collector includes areceptacle that is evacuated to a subatmospheric pressure and a fluidconduit. The receptacle includes a window and the reflector faces thewindow. The window and the reflector are exposed to the subatmosphericpressure in the receptacle. The fluid conduit extends through thereceptacle between the window and the reflector. The reflectorconcentrates solar radiation passing through the window onto the fluidconduit.

Fluid is circulated through the fluid conduit (FIG. 6, block 122). Insome examples, the fluid conduit includes a plurality of fluid channelsfor conveying the fluid, and the fluid is circulated through theplurality of fluid channels concurrently. In some examples, the fluidincludes super-critical carbon dioxide, which allows for more compactthermal collector designs and improves the solar thermal collectionefficiency with no adverse environmental impact. In these examples, eachof the fluid channels typically has a respective inner diameter between0.75 millimeter and 0.25 millimeter.

Other embodiments are within the scope of the claims.

1. A solar thermal collector, comprising a receptacle evacuated to asubatmospheric pressure and comprising a window and a reflector facingthe window, wherein the window and the reflector are exposed to thesubatmospheric pressure in the receptacle; and a fluid conduit extendingthrough the receptacle between the window and the reflector, wherein thereflector concentrates solar radiation passing through the window ontothe fluid conduit.
 2. The solar thermal collector of claim 1, whereinthe reflector comprises a pair of concave radiation-reflective surfaceportions that meet along a longitudinal axis in a longitudinal plane. 3.The solar thermal collector of claim 2, wherein the fluid conduitextends adjacent the reflector along a direction in the longitudinalplane that is parallel to the longitudinal axis.
 4. The solar thermalcollector of claim 1, wherein the reflector comprises a plurality ofreflector elements each comprising a respective pair of concaveradiation-reflective surface portions, the concave radiation-reflectivesurface portions of each reflector element meet along a respectivelongitudinal axis in a respective longitudinal plane, and the respectivelongitudinal axes are parallel.
 5. The solar thermal collector of claim4, wherein the fluid conduit comprises parallel linear segments each ofwhich extends adjacent a respective one of the reflector elements alonga respective direction in a respective one of the longitudinal planesthat is parallel to the respective longitudinal axis.
 6. The solarthermal collector of claim 5, wherein the fluid conduit comprises curvedsegments that interconnect the parallel linear segments to define aserpentine fluid flow path adjacent the reflector.
 7. The solar thermalcollector of claim 4, wherein each reflector element concentratesradiation passing through the window onto different respective sectionsof the fluid conduit.
 8. The solar thermal collector of claim 1, whereinthe receptacle comprises a base, and the window is mounted to the basewith a connection that maintains the subatmospheric pressure in thereceptacle.
 9. The solar thermal collector of claim 8, wherein the baseintegrally incorporates the reflector.
 10. The solar thermal collectorof claim 9, wherein the base is a unitary metal structure that comprisesthe reflector.
 11. The solar thermal collector of claim 10, wherein thewindow is formed of glass, and further comprising between the window andthe base a glass-to-metal seal that maintains the subatmosphericpressure in the receptacle.
 12. The solar thermal collector of claim 9,wherein the base is plastic and the reflector is bonded to the plasticbase.
 13. The solar thermal collector of claim 1, wherein the windowcomprises an antireflective coating.
 14. The solar thermal collector ofclaim 13, wherein the antireflective coating is porous.
 15. The solarthermal collector of claim 1, wherein the window comprises first andsecond parallel surfaces each of which is coated with a respectiveantireflective coating.
 16. The solar thermal collector of claim 1,wherein the fluid conduit comprises an outer surface carrying aradiation-absorbent coating, and an inner surface exposed for contactwith fluid flowing through the fluid conduit.
 17. The solar thermalcollector of claim 1, wherein the fluid conduit comprises an outersurface, and a textured inner surface exposed for contact with fluidflowing through the fluid conduit.
 18. The solar thermal collector ofclaim 1, wherein the fluid conduit comprises a plurality of fluidchannels for conveying fluid.
 19. The solar thermal collector of claim18, wherein each of the fluid channels has a respective inner diameterbetween 0.75 millimeter and 0.25 millimeter.
 20. A method ofmanufacturing a solar thermal collector, comprising attaching a fluidconduit to a base comprising a reflector; mounting a window to the baseto form a receptacle containing the fluid conduit, wherein the reflectorfaces the window and concentrates solar radiation passing through thewindow onto the fluid conduit; and evacuating the receptacle to asubatmospheric pressure, wherein the window and the reflector areexposed to the subatmospheric pressure in the receptacle.
 21. The methodof claim 20, wherein the reflector comprises a plurality of reflectorelements each comprising a respective pair of concaveradiation-reflective surface portions, the concave radiation-reflectivesurface portions of each reflector element meet along a respectivelongitudinal axis in a respective longitudinal plane, and the respectivelongitudinal axes are parallel.
 22. The method of claim 21, wherein eachreflector element concentrates radiation passing through the window ontoa different respective section of the fluid conduit.
 23. The method ofclaim 20, wherein the receptacle comprises a base, and the mountingcomprises attaching the window to the base with a connection thatmaintains the subatmospheric pressure in the receptacle.
 24. The methodof claim 20, wherein the base is a unitary metal structure thatcomprises the reflector, the window is formed of glass, and the mountingcomprises forming between the window and the base a glass-to-metal sealthat maintains the subatmospheric pressure in the receptacle.
 25. Thesolar thermal collector of claim 20, wherein the fluid conduit comprisesa plurality of fluid channels for conveying fluid.
 26. The method ofclaim 20, further comprising purging the receptacle with an inert gasbefore evacuating the receptacle.
 27. A solar collection method,comprising providing a solar thermal collector comprising a receptacleevacuated to a subatmospheric pressure and comprising a window and areflector facing the window, wherein the window and the reflector areexposed to the subatmospheric pressure in the receptacle, and a fluidconduit extending through the receptacle between the window and thereflector, wherein the reflector concentrates solar radiation passingthrough the window onto the fluid conduit; and circulating fluid throughthe fluid conduit.
 28. The method of claim 27, wherein the fluidcomprises super-critical carbon dioxide.
 29. The method of claim 28,wherein the fluid conduit comprises a plurality of fluid channels forconveying the fluid.
 30. The method of claim 29, wherein each of thefluid channels has a respective inner diameter between 0.75 millimeterand 0.25 millimeter.